BRIEF DESCRIPTION OF THE PRIOR ART
The process for methanol synthesis from synthesis gas (a mixture gas of CO and H.sub.2) containing a small amount of CO.sub.2 is a very important basic process in chemical industry and there is a standing need for improving the efficiency of the process from the standpoint of energy savings and other economics. One of the most important technologies in the field of methanol synthesis is that of providing a high performance catalyst, and as such catalysts, several ternary catalysts including the Cu/ZnO/Al.sub.2 O.sub.3 catalyst (current industrial catalysts) [e.g. Shokubai Koza, Vol. 7, ed. by the Catalyst Society of Japan, Kodansha, Jul. 20, 1989, pp. 21-39] and the Cu/ZnO/SiO.sub.2 catalyst (Japanese Kokai Tokkyo Koho No. 39287/1988), among others, are known.
Recently, methanol synthesis from CO.sub.2 and H.sub.2 as main raw materials has been a focus of attention from the standpoints of recycling of the carbon resources and of conservation of global environment. In the synthesis of methanol from a CO.sub.2 -rich feed gas, the thermodynamic equilibrium of the reaction and the reaction-inhibitory effect of by-product water accompanying the formation of methanol (Applied Catalysis A: General, 38, 1996, pp. 311-318) require for the use of a catalyst having higher activity than the conventional catalyst used in said methanol synthesis from synthesis gas. Furthermore, in the synthesis of methanol from a CO.sub.2 -rich feed gas, the deactivation of catalyst which is apparently attributable to the by-product water is remarkably great as compared with said synthesis of methanol from synthesis gas. Therefore, a demand exists for a catalyst with a much longer catalyst life than the catalyst used in the methanol synthesis from synthesis gas. This is because the ternary catalysts used conventionally for methanol synthesis from synthesis gas are insufficient in catalyst performance for methanol synthesis from a CO.sub.2 -rich feed gas.
In view of the above circumstances, some of the inventors of the present invention previously developed several copper-based multi-component catalysts including a copper/zinc oxide/aluminum oxide/zirconium oxide catalyst and a copper/zinc oxide/aluminum oxide/zirconium oxide/gallium oxide catalyst, among others (e.g. Japanese Kokai Tokkyo Koho No. 39755/1995, Japanese Kokai Tokkyo Koho No. 312138/1994, Applied Catalysis A: General, 38 (1996), pp. 311-318). The present invention represents a further breakthrough in the art including of those prior inventions. In the field of technology relating to copper-based multicomponent catalysts to which the present invention belongs, the following patent applications are known.
(1) Japanese Kokai Tokkyo Koho No. 254414/1994 discloses a method for producing a methanol synthesis catalyst from precursors of copper oxide and zinc oxide and a precursor of zirconium oxide or/and of aluminum oxide and containing 1-30 weight % of CO.sub.2 equivalent. The salient feature of the invention described in this official publication resides in the use of a catalyst precursor formulation containing a suitable amount of a CO.sub.2 source while the per se known basal composition for a methanol synthesis catalyst is retained. When a carbonate or a hydrogen carbonate is used as the precipitant in the step of precipitating copper and zinc components from an aqueous solution containing a water-soluble copper salt and a water-soluble zinc salt, the very precipitant serves as a CO.sub.2 source but when the precipitant does not contain carbonic acid, a calculated amount of CO.sub.2 gas must be blown into the catalyst formulation. It is stated in the above publication that where necessary one or more precursor components selected from among oxy acids of silicon, boron, chromium, vanadium, magnesium, and phosphorus, inclusive of salts thereof, may be added but only the use of boric acid is displayed in the examples. Regarding the calcination temperature, there is no particular description but there are references to the drying operation performed at 280.degree. C., 200.degree. C., and 300.degree. C. in the working examples and at 400.degree. C., 220.degree. C., and 300.degree. C. in the comparative examples. The temperature for reduction of catalyst precursors is said to be 100-300.degree. C., preferably 120-200.degree. C. As to the starting gas composition for use in methanol synthesis, only the case of using a gas of the composition H.sub.2 67.4%, CO 24.0%, CO.sub.2 6.6%, CH.sub.4 1.5%, and N.sub.2 0.5% is shown. This publication does not contain an example of methanol synthesis from a CO.sub.2 -rich synthesis gas and no information is available on catalyst activity and life.
(2) Japanese Kokai Tokkyo Koho No. 27645/1989 discloses a catalyst suitable for carbon oxide conversion (shift reaction and methanol synthesis) which comprises copper metal and zinc oxide and/or magnesium oxide and the metallic copper component of which has a surface area of at least 70 m.sup.2 per gram copper. It is mentioned that this catalyst may optionally contain the element X selected from among Al, V, Cr, Ti, Zr, Tl, U, Mo, W, Mn, Si, and rare earth metals in a proportion of 2-50%, particularly 4-30%, relative to the total amount of Cu, Zn, Mg, and X. In the examples, only the use of Al in combination with Zn is shown for X. As to calcination temperature, it is stated that calcination is carried out at a temperature over 250.degree. C. and generally at a temperature within the range of 300-350.degree. C. Regarding the temperature for the reduction to metallic copper, it is mentioned that the reduction is carried out at a selected temperature not exceeding 250.degree. C. and preferably not higher than 200.degree. C. With regard to the starting gas composition, only the composition H.sub.2 O 33.3 vol. %, N.sub.2 14.8 vol. %, CO.sub.2 6.1 vol. %, CO 5.7 vol. %, H.sub.2 33.3 vol. % is used in the examples. This official publication does not contain an example concerning the synthesis of methanol from a CO.sub.2 -rich gas, either, and no information is available on catalyst activity and life.
(3) Japanese Kokai Tokkyo Koho No. 32949/1984 referred to above (Japanese Patent Publication No. 39287/1988) discloses a methanol synthesis catalyst comprising copper oxide, zinc oxide, and silicon oxide. The atomic ratio to metals of silicon is 0.05-10%, preferably 0.1-3.5%. As to the silicon component, it is mentioned that any of silicon oxide sol, hydrogel, xerogel, and aerogel can be used as the starting material, with the hydrogel providing a particularly remarkable effect, that said hydrogel can be prepared typically by adding alkali to an acidic solution of a water-soluble silicate, and that the silicate may for example be sodium silicate or potassium silicate, with sodium silicate being particularly preferred. In the examples, an aqueous solution of sodium silicate (Examples 1, 3, 4), silica hydrogel (Example 2), silica xerogel (Example 5), and silica aerosol (Example 6) are used. It is stated that calcination can be carried out in an oxygen-containing atmosphere at a temperature of at least 300.degree. C., preferably 330-400.degree. C. (370.degree. C. in the examples). However, the catalyst disclosed in this official publication is limited to the Cu/Zn/Si ternary oxide catalyst and, as to the composition of the starting gas for methanol synthesis, only the use of a gas of the composition H.sub.2 70%, CO 23%, CO.sub.2 3%, CH.sub.4 3.5%, N.sub.2 0.5% is mentioned.
(4) Japanese Kokai Tokkyo Koho No.97048/1985 discloses a catalyst for use in the synthesis of fuel alcohol, which comprises copper oxide, zinc oxide, an oxyacid salt of phosphorus and/or silicon oxide, and at least one compound selected from among potassium, rubidium, and cesium. The Cu/Zn ratio is 0.2-3, and the Si/Zn ratio is 0.001-0.07, preferably 0.005-0.05. As the silicon component, it is stated that any of silicon oxide sol, hydrogel, xerogel, and aerosol can be used as a raw material, with the hydrogel providing a particularly beneficial effect, that said hydrogel can be prepared typically by adding alkali to an acidic solution of a water-soluble silicate, and that said silicate may for example be sodium silicate or potassium silicate, with sodium silicate being particularly preferred. In the examples, a silica hydrogel slurry (Example 2) or a silica hydrogel (Example 6) is used. It is also stated that calcination can be carried out in an oxygen-containing atmosphere at a temperature of at least 300.degree. C., preferably 330-400.degree. C. With regard to the starting gas composition, the composition CO 22.8%, CO.sub.2 6.5%, CH.sub.4 1.5%, N.sub.2 1.2%, H.sub.2 68.0%, the composition N.sub.2 1.6%, CO 31.7%, CH.sub.4 2.6%, CO.sub.2 9.3%, H.sub.2 54.8%, and the composition N.sub.2 0.8%, CO 21.8%, CH.sub.4 1.5%, CO.sub.2 6.1%, H.sub.2 69.8% are shown in the examples.
As mentioned above, the state of the art still requires a high performance catalyst which must possess high activity and a long life for the synthesis of methanol from a CO.sub.2 -rich gas.
When a gas composed predominantly of CO.sub.2 is reacted with H.sub.2 on a catalyst to synthesize methanol, the thermodynamic equilibrium of the reaction demands that the reaction be carried out at a lower temperature than said methanol synthesis starting with synthesis gas. Therefore, as pointed out hereinbefore, the catalyst for use must have higher activity than the activity of the catalyst used in the synthesis of methanol from synthesis gas.
Meanwhile, regardless of whether methanol is synthesized from a CO.sub.2 -rich, CO-lean feed gas or, as practiced today, from a CO.sub.2 -lean, CO-rich feed gas, the catalyst is deactivated to reduce its productivity for methanol as the reaction is continued for a long time. In order to sustain the productivity for metahnol, therefore, it has been attempted to use a large-sized reactor, to increase the reaction temperature, or to perform a periodic catalyst exchange at a higher frequency but such measures result in a considerable waste of energy and a substantial addition to production cost.
Therefore, a catalyst possessing a higher activity and a longer life is required but, as mentioned above, there has not been developed such a high-activity, long-life catalyst.
It is indeed a goal that must be reached in a very near future to find industrial catalysts that would contribute to the recycling and reutilization of carbon resources and, at the same time, be ecofriendly. The inventors of the present invention did intensive investigations to develop industrially useful high-activity, long-life catalysts for contributing to the protection of the earth's ecology and have developed the present invention.
The present invention, developed under those circumstances, has for its object to provide a copper-based catalyst possessing a high activity and a long life and a method of producing the catalyst.